Projection displays are used widely in many applications. A typical projection display consists of an illumination system that consists of a nearly point light source 1, optics 2 for the collimation of the light source, a polarizing beam splitter 4 that separates the input and output light onto/from the an imager 5 that modulates the transmittance or reflectance of the input light, and a projection lens 6 that projects the said images onto a screen 7. For an imager that works by polarization manipulation, a means 3 for the conversion of said light source into a single polarization is needed, adding to the complexity of the system. The polarization converter 3 often includes a device which may be separate or the same to homogenize the input light. An optional optical system for the separation of the input light source 1 into three separate red, green and blue primary colors is often times used to provide full colors. In this case, three separate imagers are provided to modulate the transmittance or reflectance of the three primary colors. In most such systems, a color recombination system is combined with the color separation system before the projection lens 6. Many systems for the color management of projection systems have been proposed.
In a conventional projector, the light source is a pseudo point source in the form of an arc. This arc has a finite dimension which can be 1–3 mm long and 1 mm wide. Moreover, the quartz enclosure gives rise to unwanted focusing and defocusing effects, making the assumption of a point source impossible.
Without a point source, the etendue of the optical system becomes large and is not favorable to good optical efficiency. The efficiency of such a projection system is therefore rather unsatisfactory. Typically less than 20% of the light output from the light source is projected onto the screen. For a 120 W arc lamp, for example, only 1200 lm of the 8000 lm output from the lamp is utilized. While most of the light is lost due to the large etendue of the illumination system compared with that of the imager, there is also significant loss of light in the matching the geometric shape of the point light source, usually circular, to the form factor of the imager which is usually rectangular. The form factor of the imager can take on various values of 4:3 or 16:9 depending on the source of the video signal. Television is usually in a 4:3 format while movies are in the 16:9 format. Either way, much light is lost due to the geometric shape of the projection light source.